I. Field of the Invention
The present invention relates generally to wireless communication systems and, more specifically, to a system and method for determining the position of a user terminal that communicates with an earth orbit satellite. Still more specifically, the invention relates to a system and method for measuring the round trip delay on the access channel between a gateway and a user terminal.
II. Description of the Related Art
There is an increasing need in the wireless communications environment for mobile phone location information. For example, with the advent of satellite telephone communications capabilities, it is important to determine the location of a user terminal (the mobile phone) for various reasons including billing and/or geopolitical boundaries. For example, position is needed to select an appropriate ground station or service provider (for example, a telephone company) for providing communication links. A service provider is typically assigned a particular geographic territory, and handles all communication links or calls with users located in that territory. A similar consideration arises when calls must be allocated to service providers based on political boundaries or various contractual relationships.
One industry in particular in which one can see the importance of position information is the commercial trucking industry. In the commercial trucking industry or delivery business, an efficient and accurate method of vehicle position determination is in demand. With ready access to vehicle location information, the trucking company obtains several advantages. The trucking company can keep the customer apprized of location, route and the estimated arrival time of payloads. The trucking company can also use vehicle location information together with empirical data on the effectiveness of routing, thereby determining the most economically efficient routing paths and procedures.
In the past, vehicle location information has been communicated to the trucking company home base by the truck drivers themselves, via telephones, as they reach destinations and stopovers. These location reports are intermittent at best, because they only occur when the truck driver has reached the destination or stopover and can take the time to phone the trucking company home base. These location reports are also quite costly to the trucking company because in effect they cause substantial down time of the freight carrying vehicle. This down time is due to the fact that to make a location report, the tractor driver must remove his vehicle from route, find a telephone which he can use to phone the home base, and take the time to make the location report. This method of location reporting also leaves room for substantial inaccuracies. For example, truck drivers may report incorrect information either mistakenly or intentionally, or report inaccurate estimates of times of arrival and departure.
Presently, the commercial trucking industry is implementing versatile mobile communication terminals for use in their freight hauling tractors. These terminals are capable of providing two-way communication between the trucking company home base and the truck. Typically, the communications are via satellite between the truck and a network communications center or hub.
Using the radio communication capabilities at each mobile terminal to provide vehicle position determination offers great advantages to the commercial trucking industry. Location reports would no longer be intermittent because the trucking company home base could locate a vehicle at will. No down time of the freight hauling vehicle would be required because the communications necessary for determining location could take place while the truck is en route. Also, inaccuracies in location reports would be virtually eliminated because the trucking company home base would be almost instantaneously ascertaining accurate vehicle location information.
However, using the radio communication capabilities at mobile terminals to provide a vehicle or user position is difficult when both the satellite and the vehicle continuously change their position. That is, when low or medium Earth orbiting (LEO or MEO) satellites are used for transferring signals, and when the user or vehicle changes location rapidly or frequently. Due to the orbit of the satellite and the movement of the vehicle, the range between them continuously changes. This makes it difficult to accurately measure the range between the satellite and the mobile phone, and ultimately the location of the phone on the earth""s surface. This problem is further discussed below in an example involving two objects that communicate with each other.
Generally, the range between two objects that communicate with each other can be determined in the following way. The first object transmits a first signal and notes the time of transmission. The second object receives the first signal and immediately transmits a second signal. The first object receives the second signal and notes the total time elapsed between the transmission of the first signal and the reception of the second signal. The first object then determines the round trip delay RTD from the relationship RTD=cD/2, where c is the speed of light and D is the total time elapsed between the transmission of the first signal and the reception of the second signal. The range between the two objects can then be determined from RTD.
Unfortunately, this simple relationship (RTD=cD/2) yields an accurate value of R only if (a) the two objects have fixed positions; and (b) the oscillators of both the sending and receiving units are known and stable. In other words, if one of the objects is moving relative to the other object, and/or the oscillator of one of the transmitters is inherently unstable, the simple relationship does not yield an accurate result. Thus, if the first object is a moving object, such as an orbiting communication satellite, and the second object is another moving object, such as a mobile phone mounted on a vehicle, this relationship does not yield an accurate result. Due to the orbit of the satellite and the movement of the mobile phone, the range between the two changes during the time period D. In this scenario, R1 is the range between the satellite and the mobile phone at the time the satellite transmits the first signal and R2 is the range at the time the satellite receives the second signal. Needless to say, it is difficult to determine the actual ranges R1 and R2 between the mobile phone and the satellite. The ranges can be determined as a function of the round trip delay RTD of signals between a gateway and a mobile phone. A mechanism is therefore needed to accurately determine RTD.
Previously, since it was not possible to accurately determine either R1 or R2, which are the ranges from the satellite to the mobile phone at two slightly different time instances, from a measurement that involves their sum, it was difficult to effectively determine the position of the mobile phone. If a method to effectively determine R1 or R2 is provided, it will be possible to determine the position of the mobile phone. Using R1 (or R2), and the absolute Doppler, which is equivalent to the range-rate, the position of the mobile phone can be determined. Obtaining the true Doppler, which can be used in determining the range rate, is a subject of U.S. Pat. No. 6,137,441, entitled xe2x80x9cAccurate Range and Range Rate Determination in A Satellite Communications Systemxe2x80x9d, which is assigned to the assignee of the present invention and is incorporated herein by reference. The technique of that disclosure is only briefly described herein (see Equation 18, infra). Thus an important consequence of determining R1 and R2 is that it will then be possible to obtain the position of the mobile phone.
The present invention is directed to a system and method for determining a round trip delay of signals transmitted between first and second objects, such as a satellite and a mobile telephone, that move relative to each other. In one aspect of the invention, a first signal is transmitted from the first object to the second object. The first signal is received at the second object after a propagation delay D1, the delay D1 being the time taken by the first signal to traverse from the first object to the second object. A frequency of the first signal is measured at the second object. The second object then transmits to the first object a second signal containing a report of the measured first frequency. The second signal is received at the first object after a propagation delay D2, D2 being the time taken by the second signal to traverse from the second object to the first object. The first object measures a frequency of the second signal. The first object then determines the round trip delay from the measured delays and the measured frequencies of the first and second signals.
In another aspect, the invention is directed to determining a round trip delay of signals transmitted between first and second objects that move relative to each other, in which a first signal is transmitted from the first object. The first signal is received at the second object after a propagation delay D1. The second object then transmits second signal to the first object, which is received at the first object after a propagation delay D2. The frequency of the second signal is measured at the first object. The first object then determines the round trip delay from the measured delays and the first signal frequency, the round trip delay being a function of the range traversed by the second signal.
In a still farther aspect, the invention is directed to determining a round trip delay of signals transmitted between first and second objects that move relative to each other, in which a first signal is transmitted from the first object and is received at the second object after a propagation delay D1. The second object measures a frequency from the first signal and transmits to the first object a second signal containing a report of the measured first signal frequency. The second signal is received at the first object after a propagation delay D2. The first object then determines the round trip delay from the first signal frequency, the round trip delay being a function of the delay experienced during propagation of the second signal from the second object to the first object.
Further features and advantages of the invention, as well as the structure and operation of various embodiments of the invention, are described in detail below with reference to the accompanying drawings.